At the plant on the Boulevard de Dixmude, in 1927 production began of the Rosengart LR2 automobile which appeared on the market in 1928, at the same time as the precursor of the first BMW automobile, also an Austin 7 built under licence, was appearing in Germany. Numerous variants of the Rosengart LR2 were produced and the car remained in production at least till 1939.

The LR2 was promoted for its reliability. A production car driven by François Lecot covered 900 km (560 mi) per day for more than three and a half months until it had notched up 100,000 km (62,000 mi) without any major mishap, and in the same period an LR2 achieved a class win in 80 of the 81 sporting trials in which it participated.

From 1931 the car appeared in modified form as the Rosengart LR4, now heavily differentiated from its Austin origins, and the model's robust character was demonstrated by several long distance trials. Variations on this model would survive until the 1950s, always retaining the original short 2,200 mm (86.6 in) wheelbase. A further substantial upgrade of Rosengart's small car took place in 1938 with the appearance of the Rosengart LR42N 4CV.

In the early 1930s Rosengart teamed up with the German manufacturer Adler, offering license built copies of the Adler Trumpf and Trumpf Junior, small front-drive cars that bolstered its range. It also added a conventional rear-driven car along the lines of a stretched and widened Austin. The development of front-wheel drive models led to the elegant Rosengart Supertraction model in 1937 - which competed with larger cars like the Peugeot 402 and the Berliet Dauphine for the first time in Rosengart's short history.

Unfortunately the larger cars failed to sell in numbers sufficient to justify the investment involved, and while the small cars achieved relatively satisfactory volumes, the slim margins at the lower end of the market provided a clue as to why the mainstream French auto-makers had tended to avoid it. In 1936 Rosengart himself was in financial difficulties and he transferred the company to a new organisation, Societé Industrielle de l'Ouest Parisien (SIOP).